UV spectroscopic study of the interaction between?-,?-, and?-cyclodextrins and pyridine derivatives

The antimicrobial nalidixic acid as a probe for molecular recognition of α- and β-cyclodextrins

The inclusion of the antibacterial drug nalidixic acid (NAL) in α- and β-cyclodextrin (CD) cavities was studied using UV–vis absorption and voltammetric methods. It was corroborated that the UV absorption bands of NAL are intensified in the presence of α- and β-CDs. A pronounced decrease in the peak currents of NAL was also noticed upon the addition of α- and β-CDs. From the changes in the peak currents, it was concluded that NAL forms 1:1 inclusion complexes with the various hosts, which is also revealed in the phase solubility profile of the NAL–β-CD system, as a representative example. From voltammetric data, the logarithm of the binding constants were calculated to be 2.60 and 3.20 for α- and β-CDs, respectively. The magnitude of the formation constants, as well as the Gibbs free energies for NAL with α- and β-CDs, shows that NAL is bound more strongly to β-CD, with a more apolar cavity, than to α-CD. These observations suggest that hydrophobic interaction is the most important recognition element in the binding process. All orientations of entering NAL into the cavity have been considered herein, with the purpose of characterizing the inclusion complex of NAL with CD. Combining the experimental results and molecular modeling and energy calculations on the inclusion complexes yielded a more detailed picture of the solution structure of the complex formed between NAL and either α-CD or β-CD. It was inferred that the inclusion process can occur through the 2-methylpyridine side. An additional hydrogen bond was also found to be formed between the carboxyl group of NAL, which remains outside the β-CD cavity, and the secondary hydroxyl group of β-CD. This hydrogen bond should, therefore, be operating as an important second recognition element in the NAL–β-CD system. Key words: cyclodextrins, host–guest association, inclusion complexes, nalidixic acid, solubility, voltammetry.

Thermodynamic analysis of the binding of a hepatoprotectant drug, thioctic acid, by beta-cyclodextrin

Spectroscopic and thermodynamic studies of the binding of a hepatoprotectant drug, thioctic acid, by beta-cyclodextrin (beta-CD) have been carried out using UV-vis and pH potentiometric measurements. The UV-vis spectra and the pH of the aqueous solutions of the drug were measured (i) as a function of total drug concentration in the absence of cyclodextrin, and (ii) as a function of cyclodextrin concentration at constant drug concentration. The spectroscopic study was done at pH = 7 and 25 degrees C, while the potentiometric study was performed at several temperatures ranging from 15 to 40 degrees C. From the spectroscopic data, the molar absorption coefficient, epsilon, for the pure drug in aqueous media and the stoichiometry of the inclusion complex with beta-CD were determined. The dissociation constant, Ka, of the pure drug (which is a weak acid), and the association constants of the complexes formed by beta-cyclodextrin and both the nonionized (HTIO) and ionized (TIO-) forms of the drug, have been simultaneously determined at several temperatures from the pH data, without the necessity of working with buffered solutions. The nonionic forms are complexed by the beta-CD with higher affinity than their ionic counterparts. From the dependency of the association constants on temperature (van't Hoff analysis), the inclusion complexes formed by HTIO or TIO- and the beta-CD were found to be enthalpy driven, with a favorable enthalpic term dominant over an unfavorable entropic term. Both contributions were found to show a possible dependence with temperature (DeltaCpo not equal 0). This pattern may reveal the contribution of van der Waals interactions, hydrophobic effect, and solvent reorganization as the main driving forces promoting the complexation.